In the hitherto known or conventional laser detecting and ranging apparatus of the type mentioned above, a laser beam is emitted to the atmosphere as a transmitted beam, whereon the laser beam undergone a Doppler frequency shift due to a traveling or moving velocity of a light scattering object located at a beam focus is received. By performing a heterodyne detection on received light based on the received beam and local light, a Doppler signal is detected for determining the Doppler frequency and hence the moving velocity of the light scattering object.
In the conventional laser detecting and ranging apparatus described above, a system for detecting the Doppler shift frequency relating to the carrier frequency of light is adopted. However, with this system, coherency of the Doppler signal is low, as a result of which the time (coherent time) for which the phase of the light carrier undergone the Doppler shift is continuous is short, as is known in the art.
For example, it is known that in the case where the received beam is the scattered light from aerosol in the atmosphere, the coherent time of the Doppler signal is on the order of several microseconds. Accordingly, an attempt for improving the S/N (signal-to-noise) ratio by integrating the Doppler signal over an extended time will result in the impossibility of improving the S/N ratio by the coherent integration.
Further, in order to improve the S/N ratio, the Doppler signal has to be integrated incoherently, which will ultimately lead to the impossibility of enhancing the improvement efficiency of the S/N ratio as well.
In the conventional apparatus described above, as one of the causes for the coherent time of the Doppler signal being short, there can be mentioned the fact that the carrier frequency of light is high.
In this conjunction, in order to make available the Doppler signal of a long or extended coherent time, it is conceivable to modulate the intensity of a light signal with a modulating frequency lower than that of the light signal (e.g. modulating frequency in a microwave band) to thereby detect the Doppler frequency relating to this modulating frequency.
As the hitherto known apparatuses in which the system of intensity modulation of the light signal with the modulating frequency in the microwave band, as described above, has been adopted, there are known those which are disclosed in, for example, Patent Literature 1 (Japanese Patent Application Laid-Open No. 29032/1976), Patent Literature 2 (Japanese Patent Application Laid-Open No. 150299/1984) and Patent Literature 3 (Japanese Patent Application Laid-Open No. 25786/1990).
According to the conventional technologies disclosed in the Patent Literatures 1 to 3 mentioned above, direct detection with the aid of a photo-detector is adopted as a method of converting the received beam scattered by a scattering object into an electric signal.
Although the direct detection method such as mentioned above is advantageous over the heterodyne detection adopted in many laser detecting and ranging apparatuses known heretofore in that the reception sensitivity has no dependency on polarized electromagnetic radiation of the received beam, whereby stable reception sensitivity can be ensured, it is known that the reception sensitivity is low, to a disadvantage.
In particular, in the case where the scattering object, the object for detection, is aerosol in the atmosphere and where the traveling or moving velocity (wind velocity) of the aerosol is to be detected by receiving a feeble beam scattered from the aerosol, great difficulty has been encountered in realizing the desired reception sensitivity.